1. Field of the Invention
The present invention relates to an image processing apparatus, method and program that generates a virtual endoscopic image from a three-dimensional image representing a tubular organ.
2. Description of the Related Art
In recent years, high quality three-dimensional images representing subjects became obtainable by an advance in modalities, such as a multi-slice CT. Consequently, medical image processing techniques for generating pseudo-three-dimensional images are also being developed. A virtual endoscopic display method is an example of such techniques. In the virtual endoscopic display method, a viewpoint is set within a lumen, and a perspective projection image is generated based on the viewpoint, and displayed. When a user successively changes the viewpoint in virtual endoscopic display, virtual endoscopic display can provide images as if they are imaged by a camera of an endoscope moving in the inside of the body.
As a method for generating a virtual endoscopic image, a volume rendering method is known. In the volume rendering method, an image is visualized by assigning opacity and chromaticity to a three-dimensional image, and by performing ray-casting. When a virtual endoscopic image is generated by this volume rending method, it is necessary to appropriately set an opacity curve representing a relationship between voxel values and opacity in such a manner that the opacity of “0” is assigned to a part corresponding to a lumen, in which a viewpoint is positioned, and the opacity of “1” is assigned to a part corresponding to a wall.
Meanwhile, when a tubular organ to be observed is a bronchus or bronchi, a blood vessel or vessels, or the like, the tubular structure gradually becomes thinner toward its peripheral end, and voxel values at a root part of a lumen and voxel values at a peripheral part of the lumen greatly differ from each other. Therefore, if a virtual endoscopic image is generated by using the same opacity curve for the root part and for the peripheral part, the inner wall of the lumen may be displayed at the root part. However, there is a problem that it is impossible to display the inner wall of the lumen at the peripheral part.
To solve this problem, Japanese Unexamined Patent Publication No. 2011-212219 (Patent Document 1) proposes setting an opacity curve at a changed viewpoint position when virtual endoscopic images are generated and displayed while a viewpoint position is sequentially changed. When a histogram of voxel values in a region surrounding a viewpoint position before change is HD(v), using a voxel value as variable v, and a histogram of voxel values in a region surrounding a viewpoint position after change is Hn(v), a degree of similarity between a histogram represented by HD(v−x) and histogram Hn(v) is calculated while the value of x is changed. Further, the value of x when the degree of similarity is the highest is determined as movement amount m. Further, opacity curve O(v) (=OD(v−m)), which is obtained by moving opacity curve OD(v) at the viewpoint position before change by movement amount m, is set as the opacity curve at the viewpoint position after change.